1. Field of the Invention
The present invention relates to the field of wireless telecommunications and, more particularly to a multi-diversity synchronization technique for improving synchronization performance in a wireless telecommunications application.
2. Description of the Related Art
In many wireless standards such as the TIA standard IS-136 format based on Time Division Multiple Access (TDMA) using Differential Quadrature Phase Shift Keying (DQPSK) modulation, a radio receiver is required to be able to derive timing synchronization from information contained in the received signal. Often there are a small number of data symbols contained in the transmitted signal that are known a priori to the receiver. The receiver may then use its knowledge of these symbols to locate them in the received transmission.
FIG. 1 illustrates a typical frame 10 within a transmitted signal according to the TIA standard IS-136 format. Each frame 10 is forty milliseconds in duration and consists of 1944 bits or 972 symbols (i.e., each symbol comprises 2 bits). Each frame 10 contains six time slots 20 of equal duration and size (i.e., 162 symbols), which include data from a plurality of signals transmitted by different mobile units to a base station or transmitted to a plurality of mobile units from the base station. Each full rate traffic channel uses the data fields in two time slots 20 (e.g., slots 1 and 4, 2 and 5, or 3 and 6) and thus, each frame supports three full rate traffic channels. A mobile unit will transmit or receive bursts of information, which are each allotted a time slot 20 in a frame 10, whereby a plurality of mobile units can transmit or receive over a given channel.
The format of a time slot 20 varies depending on whether it is transmitted by the base station or by a mobile unit. FIG. 2a illustrates the time slot 20 format of a signal transmitted from a mobile unit to a base station. The number of bits of each field are represented within parenthesis. The G field (6 bits) represents a guard time used to separate the present transmission burst from the previous burst. The R field (6 bits) represents a ramp time necessary to fully activate the radio transmitter. The three DATA fields (one field of 16 bits and two fields of 122 bits) hold the channel data, such as voice information. The SYNC field (28 bits) contains a sequence of symbols, which has been chosen to have good correlation properties and may be used, for example, in synchronization, equalizer training and slot identification. The SACCH (Slow Associated Control Channel) field (12 bits) is a signaling channel for transmission of control and supervision messages between the mobile units and the telecommunications system. The CDVCC (Coded Digital Verification Color Code) field (12 bits) is used to distinguish the current traffic channel from traffic co-channels.
FIG. 2b illustrates the time slot 20 format of a signal transmitted from a base station to a mobile unit. The number of bits of each field are represented within parenthesis. The SYNC, SACCH and CDVCC have the same number of bits and functions as described above with reference to FIG. 2a. There are only two DATA fields, however, each containing 130 bits, but they are still used to hold channel information, such as voice information. There is a RSVD field (1 bit) and a CDL (Coded Digits Control Channel Locator) field (11 bits), which may be used by the mobile unit to assist in the location of a Digital Control Channel.
Thus, in the TIA standard IS-136 format, there are twenty-eight known bits, or fourteen symbols, in each SYNC field (collectively referred to herein as the xe2x80x9csynchronization wordxe2x80x9d or xe2x80x9csync wordxe2x80x9d for short) that may be used by the receiver to determine time synchronization. Typically, the receiver would correlate the received signal with the a priori known sync word and use the location of the maximum correlation to determine the absolute timing reference. Alternative measures of determining the similarity between the received signal and the known sync word such as the conventionally known Mean Squared Error (MSE) based metrics can also be used as the basis for locating the sync word in a received signal.
There are conflicting design goals that impact the selection of the contents of the sync word for a telecommunications system design. For reliable synchronization performance, the sync word should have good correlation properties (i.e., an autocorrelation function equal to zero for time offsets other than zero) and be as long as possible to average out the effects of noise in the synchronization process. For efficiency purposes, it is desirable to waste as few bits as possible on non-information carrying overhead symbols such as the symbols contained in the sync word. For practical real-world systems, the synchronization word is short enough that synchronization errors are a real concern, particularly in a fading environment where the effective Signal-to-Noise Ratio (SNR) can be locally very poor during a deep fade.
The consequences of incorrectly determining the position of the sync word can be severe. When a sync word location is incorrectly determined in a burst transmission based TDMA system like IS-136 the entire frame of data contained in the mis-synchronized burst can be lost causing a significant degradation in Bit Error Rate (BER) or Frame Error Rate (FER) performance. This results in poorer speech quality in voice applications, or reduced throughput in data applications. The problem can be particularly troublesome in a high mobility fading environment where the entire contents of an otherwise acceptable frame of data can be lost if a deep fade happens to hit the sync word symbols. With denser constellations such as, those being proposed in the IS-136+ enhancements to the TIA standard IS-136 format where coherent 8-PSK (Phase Shift Keying) modulation will be used, the tolerance for synchronization errors will be even less due to the reduced margin between adjacent symbols. Fractional symbol timing errors that caused little degradation with a widely spaced 4-point constellation will have a much more significant impact on the denser 8-point constellation of 8-PSK as the eye opening is not nearly as wide.
Wireless receivers designed to work in a fading environment, particularly base stations, often take advantage of antenna diversity (i.e., use of several antennas spaced apart for receiving diverse signals) in order to mitigate the effects of fading. Antenna diversity works by exploiting the fact that it is unlikely that fades will occur at the same time in all received signals (assuming the antennas are spaced far enough apart so that the fading processes are effectively independent), and hence there is better information in the combined signal than in any individual signal. As known in the art, Maximal Ratio Combining (MRC), where the diverse signals are weighted proportionally to their received energy, is an example of a method for combining multiple antenna signals to improve receiver performance.
A high level functional block diagram of a typical synchronization scheme 50 is illustrated in FIG. 3. Baseband signals r0 to rLxe2x88x921 are received from signal receiving circuitry (not shown). Each baseband signal r0 to rLxe2x88x921 was originally received as a radio frequency (RF) signal from its respective antenna and converted by the signal receiving circuitry into digitized baseband signals (having complex in-phase I and quadrature Q components). Here the sync word positions are independently located in their individual diversities by separate single diversity sync locators 520 to 52Lxe2x88x921 (where L is the number of antennas or diversities). Once the locations S0 to SLxe2x88x921 of the sync words have been determined, the baseband signals r0 to rLxe2x88x921 may be combined in a multi-diversity demodulator 54 that can make use of the combined information found in the multiple diversities to output information OUTPUT BITS to be used by the remainder of the receiver.
Synchronization schemes, such as the one illustrated in FIG. 3, have not made use of antenna diversity since it is necessary that the signals from the multiple antennas be time aligned and coherent before they can be combined. That is, the sync word in each of the individual signals must be made to line up in time. Moreover, for phase-only modulation systems, the phase errors attributable to the fading phenomenon must be taken into account so that the multiple received signals can be combined in a constructive manner. If amplitude modulation is involved, such as when Quadrature Amplitude Modulation (QAM) constellations are used, the instantaneous amplitude error attributable to fading must also be compensated for prior to combining the multiple signals.
Currently, antenna diversity techniques are not being applied to the synchronization process. This is due to an apparent conflict that would seem to imply that antenna diversity cannot be applied to help synchronization performance because of the need to have the signals already synchronized before they can be combined. It is desirable, however, to use an antenna diversity technique (hereinafter referred to as a xe2x80x9cmulti-diversityxe2x80x9d technique) in the synchronization process to help synchronization performance in wireless telecommunications applications, such as TDMA applications.
In view of the foregoing shortcomings, and for other reasons, the present invention is directed to conducting time synchronization on received signals in a manner that reduces potential synchronization errors typically attributable to fading environments and the use of small synchronization words. The present invention comprises a method and apparatus utilizing a multi-diversity synchronization technique for improving time synchronization in wireless applications, such as TDMA applications.
In one aspect of the present invention, a method of performing time synchronization on a plurality of received signals in a telecommunications system is provided. The method includes the steps of: selecting one of the plurality of received signals to be a reference signal; estimating respective cross-correlation functions between the reference signal and each received signal not being used as the reference signal; determining respective offsets between the reference signal and each received signal not being used as the reference signal; time aligning the plurality of received signals; determining a first synchronization word location from the time aligned signals; and determining a synchronization word location for each received signal based upon the first synchronization word.
In another aspect of the present invention, a method of demodulating a plurality of received signals in a telecommunications system is provided. The method includes the steps of: determining respective synchronization word locations for each received signal using a multi-diversity synchronization method; and demodulating the received signals based upon the determined synchronization word location for each received signal.
In yet another aspect of the invention, a receiver for a telecommunications system is provided. The receiver includes a controller, said controller receiving a plurality of signals, each of the plurality of received signals comprising a synchronization word, said controller for selecting one of the plurality of received signals to be a reference signal; estimating respective cross-correlation functions between the reference signal and each received signal not being used as the reference signal; determining respective offsets between the reference signal and each received signal not being used as the reference signal; time aligning the plurality of received signals; determining a first synchronization word location from the time aligned signals; and determining a synchronization word location for each received signal based upon the first synchronization word.
In yet a further aspect of the invention, a base station for a telecommunications system is provided. The base station includes a plurality of antennas, and a receiver, said receiver coupled to said plurality of antennas, and receiving signals from each antenna, each of the plurality of received signals comprising a synchronization word, said receiver comprising a controller, said controller for determining respective synchronization word locations for each received signal using a multi-diversity synchronization method; and demodulating the received signals based upon the determined synchronization word location for each received signal.
It is an object of the present invention is to provide an apparatus for improving time synchronization performance in a wireless telecommunications application using a multi-diversity synchronization technique.
It is a farther object of the present invention is to provide a method for improving time synchronization performance in a wireless telecommunications application using a multi-diversity synchronization technique.